I've left an unsuitable shop after a year - not a pleasant experience but certainly an elucidating one, I think I'd know the signs much better now. That's not to say any place is exactly perfect but I have some ability to judge the upside versus the downside.

I've tended to find 4 - 6 years has put me to the edge of (my perceived) the value plateau - I'm not feeling I'm adding anything new or getting anything interesting out of doing the job any more. I'm not sure why it gets to that point - but I have always seemed to find myself pissed off with doing the same old crap day in day out. I'm over 6 years now though not totally sure about whether that is the case right now or at least whether it is worth trying something else.